Arson investigators said a 3-acre grass fire that initially threatened homes near Ninth and Pennsylvania streets in San Bernardino Wednesday afternoon started suspiciously.

San Bernardino City firefighters were called at 2:34 a.m. and at first saw only heavy smoke. Then they spotted a “quick moving fire in a grassy field adjacent to some houses and immediately requested additional resources,” said fire engineer Jason Serrano.

It took 23 firefighters nearly 25 minutes to douse the flames. No homes were damaged.